This invention relates to electronic musical instruments, and more particularly relates to such instruments in which the frequency of the resulting notes can be widely varied over small increments.
Due to its advantages in reliability, size and price, designers of electronic musical instruments increasingly have sought to employ digital circuitry in order to generate musical notes. Many of the resulting instruments are electronic organs which produce fundamental tones ranging from C1 having a frequency of about 32 Hz. to C8 having a frequency of about 4,096 Hz. Although the human ear is capable of distinguishing several thousand frequencies within the range from C1 to C8, electronic organ-type musical instruments are designed to synthesize only about 85 distinct frequencies within this range.
At least two different techniques have been employed to generate the required frequencies. According to the first technique, a top octave synthesizer employs a high-frequency oscillator and a series of dividers in order to generate the twelve chromatic tones in the top octave of the instrument (e.g., C7 to C8). The remaining tones are then produced by dividing the twelve outputs of the top octave synthesizer by an integer power of two. The pulses produced by the divider networks then are shaped by filters or other analog circuits in order to produce a sound of a desired characteristic or timbre.
The second technique requires the storage of a complex waveform in a digital memory and the sampling of the waveform according to the frequency of the desired note. One example of this technique is described in U.S. Pat. No. 3,515,792 (Deutsch-June 2, 1970). According to the Deutsch Patent, the digital memory is sampled by a ring counter which receives its input from a conventional top octave synthesizer.
According to U.S. Pat. No. 3,743,755 (Watson-July 3, 1973), a digital memory storing a complex waveform also can be sampled by an address decoder which decodes the number stored in a sample point address register. The sample point address register is advanced by a predetermined phase angle increment upon the receipt of each clock pulse. By varying the value of the phase angle increment, the memory is addressed at a faster or slower rate.
Although both of the foregoing techniques are adequate for the synthesis of tones in keyboard instruments which require only a few frequencies, they are not acceptable for generating a large number of frequencies from C1 to C8 where the increment between the frequencies is small (e.g., 0.5 Hz.). In order to produce frequencies over a wide range in small increments, the foregoing techniques would require either extremely large registers and counters or unrealistically high clock rates. For example, in order to achieve results similar to those obtained from the embodiment described in this document, conventional techniques would require approximately 10-bit counters and a 32 megahertz clock rate.
Although keyboard instruments are not called upon to produce a large number of frequencies over their range, other instruments, such as violins, are capable of producing a nearly infinite number of discrete frequencies within their range. In order to produce the variable frequency characteristics of these instruments, it is important to have a note oscillator capable of generating frequencies over a wide range in narrow, controllable intervals. In addition, such a capability is important in electronic musical synthesizers in order to provide added interest to the sound being produced.
Of course, frequencies can be varied over large ranges through small increments by the use of analog circuitry. However, this approach is undesirable because of the inherent instability and high cost of analog circuitry compared to digital circuitry.
Accordingly, it is the primary object of the present invention to provide a digital technique for producing pulses over a wide range of repetition rates in small, predetermined, controllable increments.
Another object is to provide a digital note oscillator for producing pulses having frequencies which extend throughout one octave and which can be set to a large number of frequencies within the octave.
Still another object of the present invention is to provide a note oscillator of the foregoing type which is capable of cyclically performing incrementing operations over a variable range at a variable increment and for generating a carry pulse and a remainder when the range is exceeded, and for readjusting the range in proportion to the remainder in response to each carry pulse.
Still another object of the present invention is to provide a note oscillator of the foregoing type in combination with a digital calculator for automatically calculating the proper values of the range and increment so that the repetition rate of the oscillator is set at a desired value.
It has been discovered that the foregoing objects can be achieved by the use of relatively inexpensive digital circuitry arranged according to the techniques described hereafter. By the use of this circuitry, note pulses can be produced over a wide range of frequency in small increments without employing high-frequency oscillators, large memories or large registers.